Study of 𝑒+𝑒− → 𝛾𝐷𝑠+𝐷𝑠

− at BESIII

Wenjing Zhu1, XiaoLong Wang1,ChangZheng Yuan2

Fudan University1

IHEP2

2019.10.9

2019/10/9 1

Outline

• Motivation and introduction

• BESIII data samples

• General selections

• Signals and background study

• Summary and plan

2019/10/9 2

Motivations • Z(3930) was discovered by Belle in 𝛾𝛾 → D𝐷 process is asigned to be 𝜒𝑐2(2P).

• M=(3929 ± 5 ± 2)MeV/𝑐2 and Γ=(29±10 ± 2)MeV

• 𝑋∗(3860) was discovered by Belle in 𝑒+𝑒− → 𝐽 𝜓 + 𝐷𝐷 , which agrees with 𝜒𝑐0 2P . • M = (3862

+26 + 40−32 − 13

) MeV/𝑐2 and Γ=(201+154 + 88−67 − 82

) MeV

• The highest charmoniumlike vector Y(4660) state was discovered in 𝑓0(980)𝜓′ final states, where 𝑓0(980) could contain ssbar components.

• How about replacing 𝐷𝐷 with 𝐷𝑠+𝐷𝑠

− ?

• The radiative decay Y(4660)→ 𝛾𝐷𝑠+𝐷𝑠

− should be allowed, and a 𝜒𝑐𝐽(nP)–like state may exist in 𝐷𝑠

+𝐷𝑠−, since there should ssbar in 𝐷𝑠+𝐷𝑠− system.

• BESIII is taking data on Y(4660), which allows the search.

• We have Y(4360) data right now, which can be used for preliminary study.

2019/10/9 3

[Phys.Rev.Lett.96,082003] [PhysRevD.95.112003]

Z(3930)→ D𝐷 𝑋∗(3860) → D𝐷 Y(4660) Y(4360)

BESIII data/MC samples and MC simulation I

• XYZ Data(Boss 703) at Ecms=4.36GeV • Signal MC:

• 0.1 M events at √s=4.36GeV and √s=4.66GeV • The channel: • 𝑒+𝑒− → 𝛾X(3.94),(via PHSP) X(3.94)→ 𝐷𝑠+𝐷𝑠−(via PHSP) • 𝐷𝑠+ → 𝐾+𝐾−𝜋+, (via D_DALITZ)

• 𝐷𝑠− → 𝐾+𝐾−𝜋−, (via D_DALITZ)

• Inclusive MC: MC-703(hadron) at √s=4.36GeV • Bkg MC:

• 0.1 M events at√s=4.36GeV • The channel:

• 𝑒+𝑒− → 𝑌 → 𝐷𝑠∗−𝐷𝑠

+ (via HELAMP 1.0 0.0 0.0 0.0 -1.0 0.0)

Case1:

• 𝐷𝑠∗− → 𝛾𝐷𝑠−(93.5±0.7) % (via VSP_PWAVE), 𝐷𝑠∗− → 𝜋0𝐷𝑠−(5.8±0.7)%(via VSS), 𝐷𝑠∗− →

𝑒+𝑒−𝐷𝑠−(6.7±1.6)× 10−3(via PHSP), 𝐷𝑠− →anything

• 𝐷𝑠+ → 𝐾+𝐾−𝜋+ (via D_DALITZ)

2019/10/9 4

BESIII data/MC samples and MC simulation II

• Case2:

𝐷𝑠∗− → 𝛾𝐷𝑠−(93.5±0.7) % (via VSP_PWAVE), 𝐷𝑠∗− → 𝜋0𝐷𝑠−(5.8±0.7)%(via VSS),

𝐷𝑠∗− → 𝑒+𝑒−𝐷𝑠−(6.7±1.6)× 10−3(via PHSP), 𝐷𝑠− → 𝐾+𝐾−𝜋−(via D_DLITZ)

𝐷𝑠+ →anything

• 0.1 M events at√s=4.36GeV

• The channel:

𝑒+𝑒− → 𝑌 → 𝐷𝑠∗+𝐷𝑠∗−(via PHSP), 𝐷𝑠∗+ → 𝛾𝐷𝑠+(via VSP_PWAVE), 𝐷𝑠∗− → 𝛾𝐷𝑠−(via VSP_PWAVE), 𝐷𝑠+ → 𝐾+𝐾−𝜋+(via D_DLITZ)

• 0.1 M events at 𝑠= 4.36GeV

• The channel :

𝑒+𝑒− → 𝑌 → 𝛾𝐼𝑆𝑅𝐷𝑠+𝐷𝑠−(via VSS), 𝐷𝑠+ → 𝐾+𝐾−𝜋+(via D_DLITZ)

2019/10/9 5

Selection criteria I • Reconstruction:

• 𝑒+𝑒− → 𝛾𝐷𝑠+𝐷𝑠−, 𝐷𝑠+ → 𝐾+𝐾−𝜋+/𝜙𝜋+, 𝜙 → 𝐾+𝐾−

• Charged tracks: • N(trk)≥ 3

• 𝑉𝑥𝑦= 𝑉𝑥2 + 𝑉𝑦2<1 cm and 𝑉𝑧 < 10𝑐𝑚

• 𝑐𝑜𝑠𝜃 < 0.93, in the active region of MDC

• Photon(s): • E_barrel > 25 MeV or E_endcap > 50 MeV • N(𝛾)≥ 1 • >5∘ away from any good charged track • t < 700ns after T0 of one event

2019/10/9 6

Selection criteria II

• PID: • Prob(K)>Prob(𝜋) and Prob(K)>0.001 for Kaon • Otherwise, it is a pion.

• Kinematic constrain • 4C kinematic fit

• 𝑀𝑟𝑒𝑐(𝛾𝐷𝑠) constraint to Ds nominal mass

• Mass windows: • 𝑀 𝐾+𝐾−𝜋+ −𝑚(𝐷𝑠) < 15 MeV for Ds signal • 𝑀𝑟𝑒𝑐 𝛾𝐷𝑠 −𝑚(𝐷𝑠) <15 MeV for another Ds

• 𝑫𝒔∗ veto: 𝑀 𝛾𝐷𝑠 −𝑚(𝐷𝑠∗) > 15MeV and 𝑀𝑟𝑒𝑐 𝐾+𝐾−𝜋+ −𝑚(𝐷𝑠 ∗) >

15MeV

• Efficiencies： • 𝑒+𝑒− → 𝛾𝐷𝑠+𝐷𝑠−, 𝐷𝑠+ → 𝐾+𝐾−𝜋+: 𝜀 = 32.28% at Ecms=4.36 GeV

• 𝑒+𝑒− → 𝛾𝐷𝑠+𝐷𝑠−, 𝐷𝑠+ → 𝐾+𝐾−𝜋+: 𝜀 = 19.61% at Ecms=4.66 GeV

2019/10/9 7

Ds signals in M(𝐾+𝐾−𝜋+)

2019/10/9 8

M = (1968.6±0.2) MeV /𝑐2 𝜎 = 4.2 ± 0.2 MeV/𝑐2

M = (1967.8±0.1) MeV /𝑐2 𝜎 = 4.4 ± 0.1 MeV/𝑐2

PDF: Gaussian + second-order polynomial

Signal MC

Data

M = (1968.6±0.3) MeV /𝑐2 𝜎 = 4.1 ± 0.3 MeV/𝑐2

Signal MC

𝑠=4.36GeV 𝑠=4.66GeV

𝜎 ≈ 4 MeV/𝑐2 Signal region: 𝑀 𝐾+𝐾−𝜋+ −𝑚(𝐷𝑠) < 15 MeV

Ds in M(𝐾+𝐾−𝜋+) from bkgs

2019/10/9 9

Case1 𝐷𝑠∗𝐷𝑠∗ MC

Inclusive MC M = (1968.8±0.3) MeV /𝑐2 𝜎 = 4.4 ± 0.3 MeV/𝑐2

The mass resolution is similar to the one from signal MC simulation

PDF：Gaussian + second-order polynomial

Case2

M = (1968.7±0.2) MeV /𝑐2 𝜎 = 4.7 ± 0.2 MeV/𝑐2

M = (1968.7±0.1) MeV /𝑐2 𝜎 = 4.6 ± 0.1 MeV/𝑐2

𝛾𝐼𝑆𝑅𝐷𝑠+𝐷𝑠−

M = (1968.3±0.2) MeV /𝑐2 𝜎 = 4.7 ± 0.2 MeV/𝑐2

Resolutions of 𝑀𝑟𝑒𝑐(𝛾𝐷𝑠+)

2019/10/9 10

PDF: Gaussian/Crystal_ball + second-order polynomial/Agus

M = (1963.8±0.2) MeV /𝑐2 𝜎 = 9.6 ± 0.3 MeV/𝑐2

M = (1974.8±0.6) MeV /𝑐2 𝜎 = 9.7 ± 0.7 MeV/𝑐2

Signal MC

Data

𝑠=4.36GeV

M = (1976.1±0.3) MeV /𝑐2 𝜎 = 14.5 ± 0.5 MeV/𝑐2

𝑠=4.36GeV

Signal MC Signal MC

𝑠=4.66GeV

𝑀𝑟𝑒𝑐 𝛾𝐷𝑠 = 𝑀 𝐷𝑠 for signal Mass resolution increases from Ecms=4.36 GeV to 4.66 GeV, becoming

worse Mass window for another Ds:

𝑀𝑟𝑒𝑐 𝛾𝐾+𝐾−𝜋+ − 1.968 < 30 MeV, 4.36 GeV data used currently

𝑀𝑟𝑒𝑐(𝛾𝐷𝑠+) from bkgs in MC samples

2019/10/9 11

PDF: Crystal ball + Agus

Case2

M = (1964.6±0.9) MeV /𝑐2 𝜎 = 8.7 ± 0.9 MeV/𝑐2

Only bkgs from 𝐷𝑠∗𝐷𝑠 have clear Ds singal in 𝑀𝑟𝑒𝑐(𝛾𝐷𝑠+)

𝐷𝑠∗𝐷𝑠∗ MC Inclusive MC

Case1

M = (1964.3±0.4) MeV /𝑐2 𝜎 = 8.5 ± 0.7 MeV/𝑐2

Distributions of M(𝛾𝐷𝑠+)

2019/10/9 12

PDF: Gaussian + second-order polynomial

𝐷𝑠∗𝐷𝑠 MC Data

The data show clear Ds* component from e+e- -> Ds*Ds, which can be simulated.

Ds* veto: 𝑀 𝛾𝐾+𝐾−𝜋+ −𝑚(𝐷𝑠∗) > 15 MeV

𝐷𝑠∗𝐷𝑠∗ MC Inclusive MC

M = (2111.6±0.4)MeV/𝑐2 𝜎 = 3.7 ± 0.4 MeV/𝑐2

Case1 Case2

M = (2111.5±0.1)MeV/𝑐2 𝜎 = 3.9 ± 0.1 MeV/𝑐2

Data

M = (2112.8±0.5)MeV/𝑐2 𝜎 = 3.8 ± 0.5 MeV/𝑐2

𝛾𝐼𝑆𝑅𝐷𝑠+𝐷𝑠− MC

2019/10/9 13

Ds* in 𝑀𝑟𝑒𝑐(𝐾+𝐾−𝜋+)

PDF: Gaussian + second-order polynomial

Case1 Data

Second Ds* veto: 𝑀 𝐾+𝐾−𝜋+ −𝑚(𝐷𝑠∗) > 15 MeV

𝐷𝑠∗𝐷𝑠∗ MC Inclusive MC

Case1

M=(2112.6±0.6)MeV/𝑐2 𝜎 = 4.9 ± 0.4 MeV/𝑐2

Case2

M=(2112.9±0.1)MeV/𝑐2 𝜎 = 5.6 ± 0.2 MeV/𝑐2

Data M=(2115.1±0.4)MeV/𝑐2 𝜎 = 4.7 ± 0.4 MeV/𝑐2

𝛾𝐼𝑆𝑅𝐷𝑠+𝐷𝑠− MC

Energy and angle of 𝛾

2019/10/9 14

𝑐𝑜𝑠𝛾𝜃 𝑐𝑜𝑠𝛾𝜃 𝑐𝑜𝑠𝛾𝜃

E(𝛾

)

E(𝛾

)

E(𝛾

) 𝑠=4.36 GeV 𝑠=4.66 GeV

Signal MC Signal MC

Data

Signal MC

𝑠=4.36 GeV

Signal MC

𝑠=4.66 GeV

E(𝛾) is related to the mass of X and 𝑠 in signal MC

Signal in 𝑀𝑟𝑒𝑐(𝛾) in MC simulation

2019/10/9 15

𝑀𝑟𝑒𝑐(𝛾) = M(DsDs) MC simulation shows very good resolution of M(DsDs) in 𝑀𝑟𝑒𝑐(𝛾) : 𝜎 = 4.2 − 4.3 MeV/𝑐2 !!!

Signal MC

𝑠=4.36 GeV 𝑠=4.66 GeV

Signal MC

M = (3939.5±0.3)MeV/𝑐2 𝜎 = 4.2 ± 0.3 MeV/𝑐2

M = (3939.3±0.5)MeV/𝑐2 𝜎 = 4.3 ± 0.4 MeV/𝑐2

Distributions of E(𝛾) and 𝑀𝑟𝑒𝑐(𝛾)

2019/10/9 16

Inclusive MC describes most of the events observed in data. Besides bkgs from inclusive MC, 𝐷𝑠∗𝐷𝑆, 𝐷𝑠∗𝐷𝑠∗and 𝛾𝐼𝑆𝑅𝐷𝑆𝐷𝑠,

there are still room from other contributions in data. Are they events of e+e- ->𝛾+DsDs directly?

Data Inclusive MC Case1 Case2 𝐷𝑠∗𝐷𝑠∗ MC 𝛾𝐼𝑆𝑅𝐷𝑠𝐷𝑠 MC

Data Inclusive MC Case1 Case2 𝐷𝑠∗𝐷𝑠∗ MC 𝛾𝐼𝑆𝑅𝐷𝑠𝐷𝑠 MC

Data sum

Data sum

sum=inclusive MC+Case1+Case2+𝐷𝑠∗𝐷𝑠∗ MC+𝛾𝐼𝑆𝑅𝐷𝑠𝐷𝑠

Summary and plan

• Summary

• We have studied the signal MC at √s=4.36GeV and √s=4.66GeV

• We have researched for Data at √s=4.36GeV and have estimated backgrounds by inclusvie MC , 𝐷𝑠𝐷𝑠

∗ , 𝐷𝑠∗𝐷𝑠∗ and 𝛾𝐼𝑆𝑅𝐷𝑠𝐷𝑠 .

• Plan • To optimize general selections • To study more data and MC samples in different energy points.

2019/10/9 17

Back up

2019/10/9 18